l38 H. L. FAIRCHILD PLEISTOCENE GEOLOGY OF NEW YORK STATE 



dioxide content of the atmosphere can more reasonably be invoked to 

 explain the larger and more irregular changes in ancient climates than 

 for the shorter and more regular changes that caused the ice-front oscil- 

 lations. The same is true of continental elevation as a cause of colder 

 climate. 



Geologic Effects of the Ice-sheet 



erosional work 



The subject relating to glaciers that has been the cause of the greatest 

 difference of opinion is the erosive power or destructional work. The 

 writer will here not discuss seriously glacial erosion in general, but only 

 so far as it applies to New York. 4 



That mountain glaciers abrade their valleys and by moderate erosionai 

 work change the V-shape to the U-shape has long been apparent. The 

 destructive work at the head of the glacier in production of cirques is 

 fully recognized, although this is largely atmospheric effect. All argu- 

 ment for deep erosion by glaciers based on the abrasional or plucking 

 action of mountain or stream glaciers fails when applied to New York, 

 as there were no effective mountain glaciers in New England and New 

 York, at least not during the waning of the Laurentian ice-body. The 

 ice disappeared from the more elevated tracts while lingering in the 

 lowlands. 



Whatever the erosive power attributed to mountain glaciers of Norway 

 or New Zealand it can not be invoked here, as New York had no such 

 glaciers. We have to consider only the work of a continental glacier. 

 Whatever destructive effects an ice-cap may have under its central or 

 subcentral mass, it has long been admitted that it is not a vigorous ero- 

 sive agent in its border zone or dissipating belt. The district in New 

 York, the Finger Lake area, which has been used in illustration of glacial 

 valley erosion, was always in merely the outer zone, or that of predomi- 

 nant deposition by the Laurentian glacier. All students of New York 

 geology practically agree on the lack of vigorous ice erosion over all the 

 rest of the State. Those who have worked in the Adirondacks and in the 

 Champlain and St. Lawrence valleys have noted the proofs of weak 

 erosion. 5 



It has been shown by Gilbert and the writer that erosion was weak on 

 the Ontario lowland of western New York. The claim for deep erosion 

 has been only for the valleys of the Finger lakes, specially Cayuga and 

 Seneca, the claim based chiefly on anomalous topographic features. 



* For the argument in general see Bull. Geol. Soc. America, vol. 16, 1905, pp. 13-74. 

 s See New York State Museum Bulletin, No. 145, 1910, pp. 147, 171-172. 



